CN106881132B

CN106881132B - Catalyst for synthesizing ammonia

Info

Publication number: CN106881132B
Application number: CN201510933298.6A
Authority: CN
Inventors: 陈萍; 王培坤; 郭建平; 常菲; 高文波
Original assignee: Dalian Institute of Chemical Physics of CAS
Current assignee: Dalian Institute of Chemical Physics of CAS
Priority date: 2015-12-15
Filing date: 2015-12-15
Publication date: 2020-04-24
Anticipated expiration: 2035-12-15
Also published as: CN106881132A

Abstract

The invention relates to a catalyst for synthesizing ammonia, which comprises a nitrogen-containing/hydrogen-containing/nitrogen-containing hydrogen compound of a main group element, a nitride of a transition metal VIII group element and relevant carriers or additives. The invention is a novel ammonia synthesis catalyst, which shows excellent catalytic activity in ammonia synthesis reaction, especially under the conditions of relatively low temperature and low pressure (less than 30bar and less than 300 ℃).

Description

Catalyst for synthesizing ammonia

Technical Field

The invention relates to a catalyst technology, in particular to a high-activity ammonia synthesis catalyst which is suitable for catalyzing and synthesizing ammonia from a mixed gas of nitrogen and hydrogen.

Background

The energy consumption of the annual synthesis of ammonia in the world currently accounts for about 1% of the world energy consumption, which is mainly determined by the characteristics of industrially adopted catalysts. The iron-based catalyst (international ferriferrous oxide is used as a main component, and the original catalyst in China uses ferrous oxide as a main component) is mainly adopted in industry, and the catalyst uses alumina, potassium oxide, calcium oxide and rare earth oxide as promoters. The iron-based catalyst has two main characteristics: firstly, the catalyst needs to be operated under the conditions of high temperature and high pressure, so that the requirement on equipment is higher, and the energy consumption for operation is extremely high; and secondly, the iron-based catalyst is easy to be poisoned, so that the requirement on the purity of the synthesis gas is strict, and a gas purification device needs to be additionally built, thereby increasing the cost for building a factory. Therefore, with the increasing energy crisis, it is important to research the catalysts for efficiently synthesizing ammonia under low temperature and low pressure conditions.

Alkali metal and alkaline earth metal are widely used as auxiliary agents in ammonia synthesis catalysts, but most of the alkali metal and alkaline earth metal are added in the form of oxides, and although good effects are obtained, the aim of obtaining high activity of the catalyst under the conditions of low temperature and low pressure is still not solved. After the nitrogen-containing/hydrogen-containing/nitrogen-containing hydrogen compound of alkali metal alkaline earth metal is used for replacing the post oxide, the activity of the synthetic ammonia is obviously improved, and the effect is obvious particularly under the conditions of relatively low temperature and low pressure.

Disclosure of Invention

As early as 1908, there was a related patent showing LiNH at high temperature and high pressure (2400psi, 450 ℃)₂Can catalyze and synthesize ammonia, and the process can be decomposed into two steps:

4LiNH₂+H₂+N₂＝Li₂NH+4NH₃(1)

2Li₂NH+3H₂+N₂＝4LiNH₂(2)

the process of catalytically synthesizing ammonia is realized through the two steps. Recent studies have shown that LiNH₂After being compounded with transition metal, the compound can be used as a high-efficiency ammonia decomposition catalyst, particularly after being compounded with Mn, the ammonia decomposition activity can even exceed that of noble metal ruthenium, and the compound can also be used as a high-efficiency catalyst for synthesizing ammonia.

Previous studies have shown that ammonia can be generated by heating alkali and alkaline earth metal amides in a hydrogen stream, the reaction scheme being as follows:

M(NH₂)_x+2H₂＝MH_x+2NH₃(where M is Li, Na, K, Cs, x is 1; where M is Mg, Ca, Sr, x is 2)

The reaction can also be carried out in reverse, under the condition of synthesizing ammonia, H₂And NH₃Are present simultaneously, so that the same effect can be obtained in the present invention from hydrides of such main group elements as from their amino/imino compounds.

The nitrogen or/and hydrogen containing compounds of the main group elements can be loaded on a certain large-surface carrier and used for increasing the specific surface area of the catalyst, thereby improving the activity.

The catalyst for synthesizing ammonia consists of main body and additive; the main body is one or more than two of group VIII metal and nitrogen-containing compounds thereof; the additive comprises one or more than two of a carrier, a nitrogen-containing compound of a main group element, a nitrogen-containing hydrogen compound of a main group element or a hydrogen-containing compound of a main group element; the mass ratio of the catalyst body to the additive is in the range of 200:1 to 1: 100.

Metals of the following group VIII and their nitrogenatesThe compound comprises: fe. Fe_xN (x ═ 1, 2, 3, or 4), Co_xN (x ═ 1, 2, or 3), Ni, Ru, Rh, Pd, Os, Ir, and Pt.

The molecular formula of the nitrogen-hydrogen containing compound of the main group element can be described as M_xN_yH_3y-nxWherein M is one or more than two of IA, IIA and IIIA group elements, n is the chemical valence state of M, n is 1, 2 or 3, M is the chemical valence state of H, M is 1-1, and when M is 1, the molecular formula is M_xN_yH_3y-nxX is 1-3, and y is 1-3; when M is-1, the formula is M_xN_yH_nx-3y，x＝1～4，y＝0～1。

The hydrogen-containing compound of the main group elements comprises two types, wherein the molecular formula of the first type is MH_xWherein M is a combination of more than two of IA, IIA and IIIA elements, x is consistent with the chemical valence of M, and x is 1, 2 or 3; the second kind is one or more than two kinds of bimetallic compound hydrides, and the molecular formula of the compound hydrides is M_xM'_yH_ax+3yWherein M is IA, IIA group element, M' is one or more than two of IIIA group elements, a is the valence state of metal M, a is 2 or 3, x is 1, 2 or 3, and y is 1, 2 or 3. .

The nitrogen-containing compound of the main group element is M₃N_yWherein M is more than two of Li, Mg and Ca, y is consistent with the chemical valence of M, and y is 1 or 2.

The carrier is Li₂O、MgO、CaO、SrO、BaO、Al₂O₃、BN、Si₃N₄、Mg₃N₂、Ca₃N₂One or a combination of more than two of AlN, molecular sieve, carbon material and metal organic framework material.

The mass ratio of the catalyst body to the additive is preferably in the range of 200:1 to 1: 100.

For from N₂And H₂The mixed gas is directly synthesized into ammonia, and the reaction conditions of the synthesis ammonia are as follows: the reaction pressure is 1 bar-200 bar, the reaction temperature is 150-450 ℃, and the space velocity is 3000-100000 h^-1，N₂/H₂The molar ratio is 19:1 to 1: 3.

Drawings

FIG. 1 Fe at a pressure of 10bar₂The catalytic activity of N/LiH (molar ratio 1: 2) is a function of temperature.

FIG. 2 Fe at a pressure of 10bar₂N/LiNH₂(molar ratio 1: 2) catalytic activity as a function of temperature.

FIG. 3 Fe at 250 ℃ under normal pressure₂Catalyst activity of N/LiH (molar ratio 1: 2) versus space velocity.

FIG. 4 shows the comparison of catalytic activities at atmospheric pressure of Ru/MgO with KH-Ru/MgO (Ru loading of 5%, K, Ru molar ratio of 1: 1).

FIG. 5 shows the comparison of the catalytic activity at atmospheric pressure between Fe/MgO and 5LiH-Fe/MgO (Fe loading 10%).

Detailed Description

The following specific examples are presented to further illustrate the invention and are not intended to limit the scope of the invention as defined by the appended claims.

The first embodiment is as follows:

mixing 1gFe₂N and LiH are mixed according to a molar ratio of 1: 2, ball milling 24 with a planetary ball mill, placing 0.5g of sample in a quartz tube reactor, and then reacting in a reaction chamber of H₂Raising the temperature to 300 ℃ at the rate of 5K/min under the atmosphere, pre-activating for 3 days under the condition, then cooling to 200 ℃, and switching to synthetic gas, (N)₂And H₂1: 3) of the mixture, the GHSV of the mixture was 10000ml/h/g, the pressure of the mixture was increased to 10bar, and the temperature rise test was started from 200 ℃.

Example two:

example one was repeated except that LiH was replaced by an equimolar amount of LiNH₂The activity results are shown in FIG. 2.

Example three:

the same catalyst was preactivated as in example one, then introduced into a reactor in a molar ratio of 3: 1H₂，N₂Mixing the gas, keeping the temperature constant at 300 ℃, changing the space velocity of the reaction gas, and testing the speed of the catalyst for synthesizing ammoniaAnd space velocity, as shown in figure 3.

Example four:

adding RuCl₃·3H₂Dissolving O in acetone, mixing with MgO by an equal-volume impregnation method, controlling the content of metal Ru in the catalyst to be 5 wt.%, naturally airing and standing for 24H, and using H to dissolve₂After reduction, with KNH₂According to K: ru atomic ratio 1:1, ball-milling for 24 hours by using a planetary ball mill, putting 0.5g of sample in a quartz tube, and then H₂At 5K/min under an atmosphere, to 300 ℃ and pre-activated for 3h under these conditions, then the mixture is passed in at a molar ratio of 3: 1H₂，N₂And (3) cooling the mixed gas (the GHSV of the mixed gas is 10000ml/h/g) to 200 ℃ and starting to increase the temperature to test the activity. The results are shown in FIG. 4.

Example five:

weighing 0.65g of iron nonacarbonyl, dissolving in 20ml of THF to obtain solution A, weighing 2g of MgO, pouring the MgO into the solution A, carrying the iron in an amount of 10 wt.%, carrying the solution by ultrasonic treatment for half an hour, standing for 24 hours until the solvent is naturally volatilized, firstly evacuating and drying for 5 hours, then treating in an oven at 60 ℃ for 5 hours, transferring to a tube furnace, heating to 500 ℃ from room temperature at a heating rate of 2K/min in Ar airflow, and keeping at 500 ℃ for 4 hours to obtain the MgO-carried iron catalyst, wherein the mark is Fe/MgO. The prepared iron catalyst was transferred to an Ar-protected glove box, 1g of Fe/MgO catalyst and 72mg of LiH (Li, Fe atomic ratio 5: 1) were weighed, and according to the procedure in example one, the LiH composite iron catalyst was obtained by nodular graphite treatment, and labeled as 5 LiH-Fe/MgO. The Fe/MgO and 5LiH-Fe/MgO activities were tested, as per the procedure in example one, respectively, as shown in FIG. 5.

Claims

1. A catalyst for synthesizing ammonia, which is characterized in that: the catalyst consists of a main body and an additive; the main body being Fe_xN, x =1, 2, 3 or 4; the additive is one or more than two of hydrogen-containing compounds of main group elements, the mass ratio of the catalyst main body to the additive is 200:1 to 1: 100;

the hydrogen-containing compound of the main group elements comprises two types, wherein the molecular formula of the first type is MH_xWherein M is IA, IIA, IIOne or more than two of IA group elements, x is consistent with the chemical valence state of M, and x is 1, 2 or 3; the second kind is one or more than two kinds of bimetallic compound hydrides, and the molecular formula of the compound hydrides is M_xM'_yH_ax+3yWherein M is IA, IIA group element, M' is one or more than two of IIIA group elements, a is the valence state of the metal M, a is 2 or 3, x is 1, 2 or 3, and y is 1, 2 or 3.

2. Use of the catalyst for ammonia synthesis according to claim 1, characterized in that: for from N₂And H₂The mixed gas is directly synthesized into ammonia, and the reaction conditions of the synthesis ammonia are as follows: the reaction pressure is 1 bar-200 bar, the reaction temperature is 150-450 ℃, and the airspeed is 3000-100000 h^-1，N₂/H₂The molar ratio is 19:1 to 1: 3.